Various heat transfer recording methods have been known so far. Among these methods, dye diffusion transfer recording systems attract attention as a process that can produce a color hard copy having an image quality closest to that of silver halide photography. Moreover, this system has advantages over silver halide photography: it is a dry system, it enables direct visualization from digital data, it makes reproduction simple, and the like.
Among these methods, in a dye diffusion type thermal transfer recording system, a heat-sensitive transfer sheet (hereinafter also referred to as an ink sheet) containing dyes is superposed on a heat-sensitive transfer image-receiving sheet (hereinafter also referred to as an image-receiving sheet), and then the ink sheet is heated, for example, by a thermal head whose exothermic action is controlled by electric signals, in order to transfer the dyes contained in the ink sheet to the image-receiving sheet, thereby recording an image information. Three colors: cyan, magenta, and yellow, are used for recording a color image by overlapping one color to other, thereby enabling transferring and recording a color image having continuous gradation for color densities.
Recently, various printers allowing higher-speed printing have been developed and commercialized increasingly in the field of the dye-diffusion transfer recording systems. The high-speed printing is a performance desirable for shortening the time of the user waiting for printing in photo shop.
In order to prevent a thermal printer head from causing fusion of heat-sensitive transfer sheet, and to provide with a lubricating property or the like, a heat-resistant lubricating layer is disposed on the side of the heat-sensitive transfer sheet that contacts the thermal printer head. The fusion may cause a running trouble that is called sticking at the time of printing, and moreover the heat-sensitive transfer sheet may be cut off due to the fusion. Further, when the lubricating property is insufficient, an image with blur (print cockle) may be formed at the portion where cockle appears due to change in shape such as the cockle that is caused by stretch of the heat-sensitive transfer sheet at the time of printing. The thermal printer head gets to contact the heat-sensitive transfer sheet at higher temperature and higher speed owing to high speed printing than normal printing. Therefore, further improvement of properties on the heat-resistant lubricating layer has been required.
For example, JP-A-8-90942 (“JP-A” means unexamined published Japanese patent application) discloses that the sticking can be removed by containing talc having specific shot type abrasion degree in the heat-resistant lubricating layer. JP-A-8-90945 discloses that the sticking and print cockle can be removed by containing talc having impurities in a specific content defined by X-ray diffraction intensity in the heat-resistant lubricating layer.
Ordinarily, heat-sensitive transfer sheets are stored as such a product form that the heat-sensitive transfer sheet is prepared by coating a dye layer on or above a base film, followed by drying, and then once stored as a roll form (also referred to as “broad roll”), and subsequently the heat-sensitive transfer sheet is taken out from the roll and cut into a sheet having a desired width (also referred to as “product roll”), and then rewound in a roll form which is then set in a printer. As a result, the heat-resistant lubricating layer and the dye layer contact each other during storage of the broad roll, so that the dye involuntarily transfers to the heat-resistant lubricating layer. In addition, when the broad roll is processed to a product roll and stored over time, the dye again involuntarily transfers from the heat-resistant lubricating layer to the surface of a protective layer or the surface of the dye layer. Therefore, when printing is carried out under that condition, there are occurred troubles in which the involuntarily transferred dye gets to transfer on a print. Especially, when the transferred dye is a yellow dye, if the yellow dye is transferred eventually on a print, the white background of the print is turned yellow resulting in so-called stains. This is a serious defect that causes quality deterioration of the color hard copy (photography).
Further, the heat-sensitive transfer sheet capable of providing with a high transfer density at time of printing is required. The dye having a high transfer density has also a high spectral absorption, and is likely to be transferred to the heat-resistant lubricating layer. Therefore, it is required to combine improvement of transfer density at the time of printing with suppression of dye transfer to the heat-resistant lubricating layer.
Meanwhile, the heat-resistant lubricating layer containing talc is described in JP-A-8-90942. The yellow dye having a 5-pyrazolone skeleton is described in JP-B-6-19033 (“JP-B” means examined Japanese patent publication). However, the heat-sensitive transfer sheets described therein are not necessarily satisfied to address the problem in that improvement in transfer density at the time of printing is combined with suppression of dye transfer to the heat-resistant lubricating layer. Therefore, it is earnestly required to address to this problem.
Further, various methods of obtaining a high quality image have been proposed in the past. For example, specific pyrazolotriazole-series azomethine dyes are proposed as the dye (see Japanese Patent No. 3596922).
Meanwhile, it is known to dispose a heat-resistant lubricating layer containing talc on the side of the heat-sensitive transfer sheet that contacts a thermal printer head of the printer in order to prevent the thermal printer head of the printer and the heat-sensitive transfer sheet from burning, and to provide with a lubricating property between the thermal printer head of the printer and the heat-sensitive transfer sheet (see JP-A-8-90942).
However, these heat-sensitive transfer sheets are not necessarily enough to solve the problems that newly arise after further studies on the ink sheet. Thus, further improvement in transfer property of high density and fastness of dye has been required.